High-impact poly(aromatic vinyl) resins such as high-impact polystyrene (HIPS) and ABS resins (acrylonitrile-butadiene-styrene resins) are polymer alloys generally obtained by blending or grafting an elastic polymer such as a conjugated diene copolymer with or on a poly(aromatic vinyl) resin to improve the impact resistance of the poly(aromatic vinyl) resin. Such a polymer alloy has a structure that the elastic polymer is finely dispersed in a matrix of the poly(aromatic vinyl) resin, which is rigid and brittle in itself, and is rigid and excellent in impact resistance.
High-impact poly(aromatic vinyl) resins obtained by using an aromatic vinyl-conjugated diene block copolymer as an elastic polymer have heretofore been known. The high-impact poly(aromatic vinyl) resins are generally produced by polymerizing an aromatic vinyl monomer or a mixture of the aromatic vinyl monomer and another monomer copolymerizable with the aromatic vinyl monomer in the presence of an aromatic vinyl-conjugated diene block copolymer. As polymerization processes thereof, are used bulk polymerization, solution polymerization, bulk-suspension polymerization and the like.
In recent years, the high-impact poly(aromatic vinyl) resins have been widely used as housing materials for electric appliances such as televisions and air conditioners. With the progress of size-enlarging and weight-reducing technology in these housing materials, there is a strong demand for materials having the ability to form a thin wall. It is thus desired that the high-impact poly(aromatic vinyl) resins be still more improved in impact resistance.
In order to improve the impact resistance of the high-impact poly(aromatic vinyl) resins, some proposals have heretofore been made. For example, Japanese Patent application Laid-Open No. 185509/1990 discloses a method in which an aromatic vinyl-conjugated diene block copolymer having a low vinyl bond content in a conjugated diene segment is used as an elastic polymer component. However, this method is yet insufficient in improving effect on impact resistance.
In addition, this aromatic vinyl-conjugated diene block copolymer involves a problem of poor bale-forming property upon production. The bale originally means a product obtained by drying raw rubber in the form of sheet, stacking and pressing sheets into about 50-cm cubes and dusting the outsides thereof so as not to stick to each other, and is a form of product used for long in natural rubber. Synthetic rubber also is often formed into a pressed product by compression-molding the resultant synthetic rubber after a polymerization step. The pressed product is required to be firmly formed and not to easily get out of shape upon handling. In the case of synthetic rubber poor in bale-forming property, it is impossible to obtain a pressed product by compression molding, or the resulting pressed product is taken in rubber pieces by rubbing the surface of the pressed product with fingers or easily get out of shape by disintegrating it with fingers. Accordingly, rubber is required to have excellent bale-forming property from the viewpoints of productivity, form of product and handling.
In order to improve the impact resistance, it has also been known to increase the molecular weight of an aromatic vinyl-conjugated diene block copolymer. However, the aromatic vinyl-conjugated diene block copolymer obtained by this method involves a problem of poor bale-forming property upon production.
Japanese Patent Application Laid-Open No. 74209/1989 discloses an aromatic vinyl-conjugated diene block copolymer in which a molecular weight (A-Mp) corresponding to a peak in a molecular weight distribution curve of a block aromatic vinyl segment (A) as determined by gel permeation chromatography (GPC) is from 30,000 to 75,000, and a proportion of block aromatic vinyl segment portions having a molecular weight at most a third of the molecular weight (A-Mp) corresponding to the peak in the molecular weight distribution curve of the block aromatic vinyl segment (A) is 25 to 50 mol %. This document describes the aromatic vinyl-conjugated diene block copolymer as being excellent in bale-forming property. However, the aromatic vinyl-conjugated diene block copolymer has involved a problem that when the block copolymer is used as an elastic polymer component for high-impact poly(aromatic vinyl) resin, the transparency of the resin component is impaired.